Currently, there is a need for Ka-band multi-beam antennas for geostationary spacecraft, that have a large enough service area, about 12×10 degrees on the Earth's surface, with a beam width of about 0.25 degrees, with a number of subscriber beam positions of 1000-2000, and the gain is not less than 55 dBi.
At the same time, the number of active channels is approximately an order of magnitude smaller than the positions of the beams and subscribers are serviced by quickly switching active channels between positions (beam hopping) with a visit time interval of the active position no more than 125 ms (to enable voice transmission) and a visit time of 1-12 ms (data superframe length).
Such a beam width and gain, at small angles of beam deflection, can be implemented for any traditional scheme of reflector antenna with an aperture of about Ø3 m. But at the same time, due to aberration effects, there is a drop in the gain by 6 . . . 10 dB and an increase in the width of the beams to 0.5 . . . 1.0 degrees at the edges of the service area. In addition, to place the required number of fixed feeders for such a density of positions and size of the service area is almost impossible.
Such a beam width and an any number of beam positions can be realized in Active Electronically-Scanned Array (AESA), but the required gain and minimization of the grating lobes can be ensured by two mutually exclusive ways:
Or almost completely get rid of the grating lobes, which implies weakly directed partial feeders with a lattice spacing of about one wavelength. In this case there will be an insignificant, no more than 1 . . . 3 dB drop at the edges of the service area, but the lattice with an aperture of Ø3 m and a hexagonal grid step equal to the wavelength (transmission, 20 GHz) should have about 36 thousand partial feeders. With the current level of technology is almost impossible.
Or use highly directional partial feeders with a diameter of 6-8 wavelengths. But the grating with such feeders will have a gain drop at the edges of the service area, about 6 . . . 10 dB, and the grating lobes become unacceptably powerful and may even exceed the level of the main beam with large deviations. The use of an aperiodic lattice with highly directional partial feeders, for example, an annular one, somewhat improves the position with the grating lobes, “smearing” them around the annular region and reducing their level by 15 . . . 20 dB. But with extreme deviations of the beam, this annular region can still get to the surface of the Earth, which is highly undesirable. In addition, there is the problem of satellite illumination on the opposite side of the geostationary orbit.
There are various schemes of reflector antennas with an irradiating device (ID) on the basis of a phased array (Phased Array Feed Reflector, PAFR). The advantage of such schemes is that a fairly simple focusing device provides the necessary aperture, and the difficult to implement active phased array has small dimensions. Such a lattice can form multiple focal radiation centers (virtual irradiators) using certain subarrays of partial feeders.
In such an ID, the grating lobes can be almost completely removed, since, due to the much smaller area of the ID, the lattice spacing can be reduced.
They can also be significantly reduced in the far zone of the antenna, since in the zone between the ID and the focusing system, they are not a rotated flat wave front, but a rotated spherical wave front and mostly go beyond the focusing system. In addition, a certain aperiodicity of placement of partial feeders can be made by placing them on the concave spherical surface of an ID, providing approximately the same viewing angle of the focusing system for each partial feeder.
But this scheme does not eliminate the main drawback of systems with a focusing system and a point feeder. All of them have optical aberrations (mostly coma), and can realize a rather small coverage area with given beam parameters.
In the invention [JP 5014193], adopted by the authors for the prototype, an attempt was made to form virtual irradiators, to some extent taking into account the problem of aberrational distortion.
This invention has a focusing system consisting of one or a plurality of reflectors, an ID, consisting of an array of feeders, covering the radiation zone of the focusing system and located closer or further to the focal point of the focusing system, and a beamforming system controlling the amplitude and phase parameters of the feeders in the subarrays, corresponding to each ray. This invention involves measuring (or calculating) the amplitude-phase characteristics of the incoming beam for each feeder in a subarray, limited by the projection of the aperture from the incoming beam on the ID surface, and assigning these characteristics to the same feeder to form the outgoing beam.
The disadvantage of this method is that the simple definition and setting of the phase (phase shift) for each partial feeder will lead to the common problems of all phased arrays on phase shifters:                low positioning accuracy of the rays and a large phase error, since the bit depth of the phase shifters, as a rule, does not exceed 6-8 bits;        intersymbol interference, which will lead to a significant reduction in the signal bandwidth;        the dependence of the angle of deflection of the beam from the frequency, which will lead to the “spreading” of the radiation pattern along the spectrum of the modulated carrier frequency—an analogue of chromatic aberration in optics.        
However, due to the relatively small size of the lattice, these problems can be eliminated by a beamforming system with true time delays, which is supposed in this invention.
A more serious disadvantage is the lack of criteria for optimizing the geometry of the surfaces of the focusing system and the relative position of the ID and the focusing system. There is also a problem with power amplifiers of feeders for a transmitting ID with sub-arrays of feeders (to be discussed below).